A film known from U.S. Pat. No. 3,741,253 to Brax comprises a core layer of a vinylidene chloride copolymer (saran) between a layer of ethylene-vinyl acetate copolymer and layer of a cross-linked ethylene-vinyl acetate copolymer. Ethylene-vinyl acetate copolymer (EVA) has some improved properties over the previously used polyethylene. Vinylidene chloride copolymers are known barrier materials to fluids such as oxygen.
As disclosed in U.S. Pat. No. 4,064,296 to Bornstein the core layer may also be a hydrolized ethylene-vinyl acetate copolymer (EVOH). It has similar oxygen barrier properties as vinylidene chloride copolymers and offers the advantage that it may be irradiated without discoloration, which is further discussed below.
Blends of linear low density polyethylene and ethylene vinyl acetate copolymer in oriented barrier films are disclosed in U.S. Pat. No. 4,457,960 to Newsome, which claims an oriented multiple layer polymeric film, comprising (a) a first barrier layer, said first layer having two opposing surfaces; (b) a second layer adhered to one said surface, said second layer being 10% to 90% linear low density polyethylene and 90% to 10% ethylene vinyl acetate; and (c) a third layer adhered to the other said surface, the composition of said third layer being selected from the group consisting of (i) ethylene vinyl acetate, and (ii) blends of 10% to 90% linear low density polyethylene with 90% to 10% ethylene vinyl acetate.
The so called linear low density polyethylenes are copolymers of ethylene and varying amounts of higher alpha-olefins with e.g. 5 to 10 carbon atoms per molecule (U.S. Pat. No. 4,076,698) or 3 to 8 carbon atoms per molecule (Published European patent application 120503 published Oct. 3, 1984, assigned to Union Carbide), for example copolymers of ethylene and butene-1, copolymers of ethylene and octene-1, and the like. Depending on their density these materials are referred to as linear low density polyethylene (LLDPE) or very low density linear polyethylene (VLDPE), the separation line being at a density of about 0.910 g/cc. Some properties of VLDPE have been described in Plastics Technology, September 1984, page 113. In October 1984, on page 13 of Plastics Technology, was another article describing VLDPE entitled "New Kind of Polyethylene Combines Flexibility, Toughness, Heat Resistance". The article lists a number of the properties of VLDPE and compares them with EVA. VLDPE is also described in a company brochure published in February 1984 by DSM in the Netherlands and entitled "Stamylex PE". Their properties are said to be a unique combination between those of standard polyethylene and polyolefinic rubbers. Their sealability and their compatibility with other polymers has been mentioned.
U.S. Ser. No. 911,936, which corresponds to Published European patent application No. 0217252 published on Apr. 8, 1987, commonly assigned to W. R. Grace, discloses a thermoplastic, multilayer, packing film having superior heat shrink and cold seal properties. Optionally, the film has a barrier layer. The sealing layer of the film comprises a copolymer of ethylene and higher alpha-olefin said copolymer having a density less than about 0.920 g/cm.sup.3. Specifically, when using a copolymer of ethylene and butene, the comonomer content should be between about 10 and 20% by weight, based on the copolymer. Such copolymers have a density of less than about 0.915 g/cm.sup.3. When employing a copolymer of ethylene and octene, the comonomer content should preferably be increased to between about 12 and 25% by weight, based on the copolymer. Such copolymers have a density of less than 0.920 g/cc. It is also possible to blend the ethylene/alpha-olefin copolymer with up to 50% by weight, based on the sealing layer composition, of a polymer which is compatible with said ethylene/alpha-olefin copolymer. Such further polymer may preferably be selected from linear low density polyethylene (LLDPE) with a density above 0.920 g/cc, linear high density polyethylene (LHDPE), low density polyethylene (LDPE), ethylene vinyl acetate (EVA), acid modified EVA, polypropylene, ethylene/propylene copolymers, ionomeric polymers, and ethylene/alkyl-acrylate (EAA) copolymers wherein the alkyl moiety has 1 to 8 C atoms, in particular ethylene/methyl-acrylate (EMA), ethylene/ethyl-acrylate (EEA) and ethylene/butyl-acrylate (EBA). The ethylene/alkyl-acrylate copolymer which can be blended with the ethylene/alpha-olefin in the heat sealing layer can comprise about 3 to 30% by weight of alkyl acrylate. In order to achieve optimal results as per U.S. Ser. No. 911,936, the comonomer content must increase when going from the C.sub.4 alpha-olefin (butene(1)) to the C.sub.8 alpha-olefin (octene(1)).
U.S. Pat. No. 4,640,856 to Ferguson et al, commonly assigned to W. R. Grace, discloses a multi-layer, thermoplastic barrier film having at least three layers comprising: (a) a layer consisting essentially of very low density polyethylene having a density of less than 0.910 gms/cc; (b) a barrier layer comprising a material selected from the group consisting of: (1) copolymers of vinylidene chloride and (2) hydrolyzed ethylene-vinyl acetate copolymers; (c) a thermoplastic polymeric layer, said layer being on the side of the barrier layer opposite to that of layer (a); and (d) the shrinkage of layer (a) controlling the shrinkage of the entire multi-layer barrier film, said multi-layer film having been oriented and rendered heat shrinkable at a temperature below 100.degree. C. (212.degree. F.), said orientation temperature being about 40.degree. F. or more below the melt temperature of said very low density polyethylene.
U.S. Pat. No. 4,597,920 to Golike, assignor to du Pont, (July, 1986) discloses a process for making a shrink film by stretching biaxially, without prior crosslinking, a film made of a copolymer of ethylene with at least one C.sub.8 -C.sub.18 alpha-olefin, which copolymer has two distinct crystallite melting points below 128.degree. C., the difference between these melting points being at least 10.degree. C., and stretching being carried at a temperature within the range defined by these melting points.
It is an object of the present invention to provide a packaging film and bags made therefrom which have excellent or improved orientation characteristics over those of the materials used in the past. This means that the orientation speed during processing should be faster.
It is a further object of this invention to provide a packaging film and bags made therefrom which have excellent heat-shrinkability characteristics compared to those of materials used in the past.
It is a further object of this invention to provide a packaging film and bags made therefrom having the above two characteristics and also having excellent abuse resistance or strength, as compared to materials used in the past and thereby provide a minimal risk of breakages when bags made of the film material are utilized in automated loading processes.
Finally and most importantly it is an object of this invention to provide a material for films and bags combining the three above advantages, i.e. excellent shrinkability characteristics, excellent orientation characteristics, and excellent abuse resistance.